int tadd_ok(int x, int y); // return 1 if not overflow or underflow
/* Determine whether arguments
/* WARNING: This code is buggy. */
/* return 1 if not overflow or underflow */
int tsub_ok(int x, int y) {
    // return tadd_ok(x, -y); // old code error: when y is 0x80000000
    if(x>0&&y<0){
        return tadd_ok(-x,y);
    }else if(x<0&&y>0){
        return tadd_ok(x,-y);
    }else return 1;
}